Liquid chromatography and auto-sampler systems rely mostly on flat rotary based design valves to handle various fluids. Many different configurations are possible, such as the standard six-port injection valves, valves with syringe port or for sample stream selection, column selection, multi-position/multitask, such as loading, injecting, washing, etc. In most cases, valves have a flat rotor and a flat stator. The flat rotor is pushed against a fixed flat stator. The rotor has various grooves machined in it, allowing different stator port connection schemes to fit any particular application.
To exemplify the description of such a prior art valve system, we will refer to a typical six-port liquid chromatographic valve. Such valve is shown in FIGS. 1A to 2. This technology has been used for more than half a century. The sealing, i.e. the leak integrity, between two adjacent ports and all outboard leaking integrity from any port, is provided by applying a load on the rotor, such required load being greater for higher pressure operations. The load, or pressing force, is normally set by a mechanical biasing element, such as a compression spring or a stack of Belleville discs. Since the force maintaining the rotor on the stator is relatively high, so is friction and resultant wear. Because wear occurs when the rotor is turned against the stator, the lifetime of such a valve is short. Scratches eventually appear on the rotor, which is usually made of softer material than the stator.
Friction between the rotor and the stator causes particles to be generated, further increasing the problems associated with wear. Leaks are likely to appear, and eventually the valve will have to be repaired or replaced. This problem may be found in most flat and conical rotary valves available.
Referring to FIG. 3, from U.S. Pat. No. 6,643,946 pertaining to Rheodyne, a rotor and a stator of a typical flat rotary valve are shown, both presenting scratches resulting from friction and wear. In order to increase the lifetime of rotary valves, U.S. Pat. No. 6,453,946 discloses a valve in which one of the sealing surfaces is coated with Tungstene Carbide/Carbon (WC/C) while the other sealing surface is provided with a fluorocarbon polymer.
In Ultra High Performance Liquid Chromatography (UHPLC) applications, the process pressure can be as high as 20,000 PSI. By “process pressure” it is meant the pressure of the fluid circulating in the valve, such as the sample gas, carrier gas or liquid mobile phase. At such a pressure level, the required rotor loading force provided by the biasing element is high, and so is friction and resultant wear. Although coating the sealing surfaces of the stator may improve the lifetime of the rotary valve, there is a still a need for an improved valve system that may allow even longer lifetime, especially for high pressure applications.
Also known are the following references: U.S. Pat. Nos. 3,297,053; 3,640,310; 6,193,213; 6,453,946; 7,503,203; and US Patent application 20100059701.
In U.S. Pat. No. 6,193,213, the process fluid is used for applying an additional load force on the rotor. The load force is therefore a function of the process pressure, which is generally constant. As a result, the overpressure on the rotor is also constant and equally applied whether the rotor is stationary or rotating, which disadvantageously does not allow the load force to be varied. A further disadvantagemay result from using the process fluid within different sections of the valve because this increases the risk of contaminating the fluid. Additional seals are then required to properly seal the different valve sections.
Another problem arises from the fact that valves are usually tuned at ambient temperature, but are mostly used at different temperatures, from cryogenic temperatures to temperatures of around 350° C. The behavior of each part of the valve may therefore differ greatly depending on the temperature range at which it is operated. Consequently, a valve can work perfectly when tuned and operated at ambient temperature, but important leakage may occur when the valve is used in a system operated at a different temperature.
In light of the above, there is also a need for an improved valve, or an improved system for varying the load applied on the valve element of a valve. There is also a need for a method of operating a valve that would help reduce friction between the movable valve element and the stationary body of the valve.